1. Field of the Invention
The present invention relates to ink jet printing and more particularly relates to an improved ink droplet sensing method and apparatus.
2. Background
Ink jet printing is a form of non-impact printing where ink droplets are squirted through an orifice and directed to specific locations on a print medium to create an information pattern on the print medium. U.S. Pat. No. 3,596,275 to Sweet discloses one such printing system where a sequence of droplets are generated, charged, and deflected away from an initial trajectory so that they strike the print medium in an ordered pattern. Subsequent to Sweet's original work the concept of deflecting some droplets away from the medium to a gutter evolved so that even greater droplet position control is possible.
One high speed ink jet printer proposal comprises multiple ink jet nozzles spaced across the width of a print plane. Each nozzle can throw a series or sequence of ink droplets to a specific swath or portion of the print plane so that in combination the multiple nozzles can send ink drops to any position across the plane. By translating the print medium past the nozzle array the entire print medium can be selectively encoded with ink droplets to create a permanent ink jet recording.
U.S. Pat. No. 4,255,754 to Crean et al entitled "Differential Fiber Optic Sensing Method and Apparatus for Ink Jet Recorders" discloses an ink jet system configured to include multiple ink jet nozzles spaced across the print medium's width. This system includes multiple droplet charging and deflecting electrodes which interact with droplets to deflect them to appropriate picture element locations (or pixels) on the medium. In particular, each nozzle has associated with it a charging electrode and a pair of deflection electrodes which electrostatically bend charged droplets to their intended positions or to a droplet gutter.
In this configuration, it is important for proper printer performance that each picture element is addressed by one ink jet nozzle. Stated another way, the droplets from adjacent nozzles should neither overlap nor leave gaps but instead should "stitch" together across the print plane.
Techniques are known in the art for stitching together the ink jet droplets from a multiple nozzle array. One technique is accomplished with the use of a programmable computer which both monitors and controls ink jet performance. During a calibrate mode the controller causes ink drops to follow certain specific trajectories. By observing the charging voltages that must be placed on the charging electrodes to cause the droplets to follow these calibrate trajectories, it is possible for the controller to determine and control the generation of charging voltages for other droplet trajectories.
The printing system disclosed in the above referenced Crean et al patent uses optical fibers to sense ink droplets following the calibration trajectories. A drop sensing zone is defined for each sensing site in a space between the faces of a single input fiber and two output fibers. An LED light source is coupled to a remote end of the input fiber and photosensors are connected to remote ends of each output fiber. When ink droplets enter the region between the input and output fibers the output of the two photosensors changes with time and sensing circuitry connected to the photosensors gives an indication that droplets are passing the sensing site.
Since a typical ink jet printer may include many ink nozzles spaced across the paper width, there must be many sensing sites also spaced across the paper width. Each site includes two output fibers and one input fiber which must be routed away from the sensor sites to the interpreting electronics and the LED input. In an array having many such optical fibers the mechanics of mounting and routing these optical fibers becomes burdensome. Copending U.S. patent application Ser. No. 204,443 to Houston et al filed Nov. 6, 1980 and entitled "Integrated Waveguide Drop Sensor Array and Method for Ink Jet Printing System" addresses the routing problem. In that application, the use of photofabricated light paths on an underlying substrate is disclosed. While simplifying the generation of the optical paths leading to and from the optical sensing sites, the Houston et al disclosure in no way reduces the number of those paths.